User Guide

Nokia Customer Care System Module

Company Confidential RH-30/RH-31

LCD & keypad illumination

In RH-30/RH-31, white LEDs are used for LCD and keypad lighting. Two LEDs are used for

LCD lighting and six LEDs for keyboard. LCD and keypad LEDs can be controlled sepa-

rately. A step up DC-DC converter is used as a LED driver.

LCD LEDs are driven in serial mode to achieve stable backlight quality. This means that a

constant current flow is lead through LCD LEDs. The feedback signal, FB, is used to con-

trol the current. A driver will increase or decrease the ‘Vled’ output voltage for LEDs to

keep the current stable. LCD LEDs are enabled by UEM PWM output called Dlight.

Keyboard LEDs are driven in 2 serial 3 parallel mode. Serial resistance R is used to limit

the current through LEDs. The feedback signal, FB, from LCD LEDs controls also the cur-

rent of keyboard LEDs. Keypad LEDs are enabled by UEM PWM output called Klight.

Figure 26: LCD Keypad Illumination